"I want to see Supreme Court justices who are nominated uphold the Constitution," Vukmir said. "I am 100% pro-life. I want to make sure we have justices who will look at the Constitution from that perspective."

"I believe the Constitution is pro-life," Nicholson said, adding that he would "support judges that also view the Constitution as pro-life."

The two candidates are walking a fine line since the possibility of overturning Roe v. Wade is the greatest it has been in decades because of the retirement of Supreme Court Justice Anthony Kennedy.

Kennedy in 1992 joined a majority that upheld the heart of Roe v. Wade, which established the constitutional right to an abortion.

President Donald Trump, who has promised to appoint abortion opponents to the courts, has nominated Appeals Court Judge Brett Kavanaugh to fill Kennedy's seat, setting up a likely U.S. Senate confirmation vote before the fall election.

While Nicholson and Vukmir are running in a Republican primary and want to make it clear to voters they oppose abortion, they also have to keep in mind that in a general election against Democratic U.S. Sen. Tammy Baldwin, a majority of voters may not be with them.

A July Marquette University Law School Poll found that 32% of Wisconsin's registered voters wanted abortion to be legal in some cases and another 27% wanted abortion legal in all cases.

Baldwin has long supported abortion rights and is endorsed by NARAL Pro-Choice America and Planned Parenthood Action Fund.

Abortion has become an increasingly important issue in the Republican race leading to the Aug. 14 primary.

Vukmir, a state senator from Brookfield, is campaigning on her long-standing opposition to abortion. She helped push through a 2015 law that banned abortion 20 weeks after fertilization.

Vukmir backed a 2013 law that required women seeking abortions to get ultrasounds and also required doctors who performed abortions to have admitting privileges at a nearby hospital. The portions of the law having to do with admitting privileges were found unconstitutional and state taxpayers had to pay Planned Parenthood of Wisconsin and another abortion provider $1.6 million in legal fees.

Nicholson has faced attack ads from a pro-Vukmir super PAC over his past support for abortion rights. As president of the College Democrats of America, he spoke of a woman's right to seek an abortion at the 2000 Democratic Convention.

Nicholson said the ad "misrepresents" his views on the issue. He said he has become pro-life through his experiences of marriage and raising his family.

"They know the mission of the pro-life movement is to pull people to the side of protecting innocent life, and my story is representative," he said.

The two candidates received a 100% rating on answers to a questionnaire from Pro-Life Wisconsin.

In that questionnaire, the candidates agreed to vote in favor of legislation that bans abortion in all cases, including rape and incest. They also agreed to back the Sanctity of Human Life Act, which says human life begins with fertilization.

"These are very strong pro-life Senate candidates," said Matt Sande, legislative director for Pro-Life Wisconsin. Sande added, "Not only do they want to overturn Roe, they want to establish legal personhood for the pre-born child."

In early July, Wisconsin Right to Life's political action committee endorsed both Nicholson and Vukmir. That came as a surprise to some since Vukmir has long been supported by the group.

The endorsement decision wasn't taken lightly, said Heather Weininger, Wisconsin Right to Life's executive director. Weininger said she did not vote on the endorsement.

"I will tell you the vote was difficult, it was not unanimous," she said. "There was a lot of discussion over many months. Ultimately, we look beyond the voting record. What kind of resources do they have? Where does their support come from? What is our membership telling us?"

Weininger said it's not unusual for the group to support more than one candidate in a primary.

"What we want to be sure of is beating Tammy Baldwin and our ultimate goal is to support whoever makes it through this primary," she said. "We know Tammy Baldwin's record on life and we don't want her record to stay in the Senate."